1. Field
This disclosure relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same.
2. Description of the Related Art
Rechargeable lithium batteries have recently drawn attention as a power source for small portable electronic devices. They use an organic electrolyte and thereby have at least twice the discharge voltage of a comparable battery using an alkali aqueous solution, and accordingly have high energy density.
During the initial charge of a rechargeable lithium battery, lithium ions, which are released from the lithium-transition metal oxide of the positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., and a thin film on the surface of the negative electrode may be formed. This film is referred to as a solid electrolyte interface (SEI) film.
The SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of lithium ions. The ion tunnel prevents disintegration of the structure of the carbon negative electrode, which is caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode. This co-intercalation may collapse the carbon negative electrode.
Once the SEI film is formed, lithium ions do not react again with the carbon electrode or other materials, such that the amount of lithium ions is reversibly maintained. That is, carbon of a negative electrode reacts with an electrolyte at initial charge to form a passivation layer such as a SEI film on a surface of the negative electrode. Thereby, decomposition of an electrolyte may be prevented and a stable charge and discharge may be maintained.
In this way, a rechargeable lithium battery does not undergo an irreversible formation reaction of a passivation layer after an initial charge reaction and maintains a stable cycle-life. However, gases are generated due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction. These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used. Due to the gases generated inside the battery, the battery may become swollen in a thickness direction when it is charged.
When the battery is fully charged and kept at a high temperature, the organic SEI film gradually decomposes based on the electrochemical energy and thermal energy that increases as time passes, continuously causing a side-reaction with a new adjacent surface of the negative electrode with exposed electrolyte. The continuous generation of gases increases the internal pressure inside the battery. Such an increase of internal pressure causes distortion of the battery in a certain plane.